An antenna array of the generic type described herein typically has two or more primary antenna elements, but at least two antenna elements which are arranged alongside one another and one above the other, thus resulting in a two-dimensional array arrangement. These antenna arrays, which are also known by the term “smart antennas,” are also used, for example, for target tracking (radar) in the military field. The expression “phased array antenna” is also frequently used in these applications. Recently, however, these antennas are being increasingly used for mobile radio purposes as well, particularly in the frequency bands from 800 MHz to 1000 MHz, and from 1700 MHz to 2200 MHz.
The development of new primary antenna element systems has now also made it possible to design dual-polarized antenna arrays, in particular with polarization alignments of +45° and −45° with respect to the horizontal and vertical.
Irrespective of whether they are fundamentally dual-polarized antenna elements or only single-polarized antenna elements, antenna arrays such as these can be used to determine the direction of the incoming signal. At the same time, however, the emission direction can also be changed, that is to say selective beam forming is carried out, by appropriate adjustment of the phase angles of the transmission signals which are fed to the individual columns.
This alignment of the emission direction of the antenna can be provided by electronic beam swiveling, that is to say by varying the phase angles of the individual signals by suitable signal processing. It is also possible to use suitably designed passive beam forming networks for this purpose. The use of active phase shifters or phase shifters which can be driven by control signals in these feed networks is also known as a means for varying the emission direction. A beam forming network such as this may, for example, be in the form of a so-called Butler matrix which, for example, has four inputs and four outputs. Depending on which input is connected, the network produces a different but fixed phase relationship between the antenna elements in the individual dipole rows. An antenna design such as this with a Butler matrix is disclosed, for example, in prior art U.S. Pat. No. 6,351,243.
All the arrangements that have been mentioned for beam forming are subject to the problem, however, that the phase angle of the individual signals which are fed to the individual primary antenna elements depends on the length of the connecting cable. Since this may often be relatively long—particularly at exposed locations—the phase angle of the antenna generally needs to be calibrated by a calibration process that takes the connecting cable into account. Active electronic components in the individual feed lines, such as transmission or reception amplifiers are, of course, also likewise included in the calibration process.
With electronic components such as these, calibration is often required as a result of component tolerances and temperature sensitivity of the group delay time.
One specific problem relates to the use of upstream Butler matrices for direction forming. In this case, calibration is very complicated, since the phase angle downstream from the Butler matrix is not uniform and, furthermore, two or more primary antenna elements of the antenna normally receive a portion of the signal.
No appropriate calibration methods for appropriately optimized setting of a desired phase angle for the individual antenna elements are known, particularly for dual-polarized antennas.
Methods are also known in which individual elements of a vertically arranged antenna array are each fitted with probes that are connected to the dipoles. Antennas such as these are used, for example, for aircraft radio. The probes that are used in this case are used to verify that each dipole is receiving the appropriate power. The overall level is thus detected and measured by connection to one output. If a dipole is not receiving sufficient power, this defect is thus identified quickly, since the overall level then changes. Since all the primary antenna elements are interconnected by means of a common feed network, the phase angle or the delay time between the probe output (monitor output for aircraft radio antennas) and the input of the antenna is of only secondary importance.
An arrangement such as detects the power. Differentiated evaluation of the phase of the individual primary antenna elements is neither possible nor necessary in systems such as these since they comprise only a rigid array arrangement, with the elements connected to one another in a fixed manner and whose main beam direction is not varied by swiveling or switching.
U.S. Pat. No. 5,644,316 discloses an active phase variation device for an antenna, in which a coupling device is provided upstream of the antenna array. The coupling device is followed by N parallel-connected transmission paths, which each have a phase variation device and an amplitude variation device via which, on the output side, an antenna element that is associated with the relevant path is driven. In order to carry out appropriate calibration, the individual paths are measured successively, with a probe that is provided on the output side being associated with each relevant antenna element. The transmission signal which is supplied via the relevant path to the antenna element is detected via the probe and is likewise supplied to an evaluation device. By evaluation of the transmission signal that is tapped off on the input side in comparison with the transmission signal that is received via the probe, the phase and amplitude variation device which is provided in the respective path being measured can be driven appropriately via this respective path. A calibration device which is comparable to this extent has been disclosed in U.S. Pat. No. 6,046,697. In this apparatus as well, a specific signal is preferably supplied via the individual signal paths to an antenna element which is associated with the individual signal paths, in order to use a probe, which is fitted in the near field of the antenna element, to detect a phase angle signal. This can be used to drive the input side of a phase control device, via which the signal is supplied to the relevant antenna element. Instead of a probe device which can be positioned differently, it is also possible to provide coupling devices, which are then associated with each individual antenna element. The coupling devices can be connected and disconnected successively via the switching device.
A method and an apparatus for calibration of an antenna array have also been disclosed in DE 198 06 914 C2. In this exemplary embodiment as well, each antenna element has an associated directional coupling device, via which a signal can in each case be emitted from the relevant signal path. For calibration purposes, test signals are in each case sent successively to an individual antenna element, and a signal value is emitted via the directional coupler. The directional couplers are followed by a power splitter. The signal which is supplied to an individual antenna element during the calibration process is in consequence emitted via the relevant directional coupler, and is passed via the power splitter to its central port. The central port is connected to a reflection termination. The transmission signal component is reflected on this reflection section, and is split into signal elements with the same amplitude and phase at the branching ports, with the number of branching ports being the same as the number of transmission or reception paths. The individual signal elements which are derived from the transmission signal are now injected via the directional couplers into the individual reception paths. The signal elements which are produced at the outputs of the reception paths and received by the beam forming network are evaluated by a control device. This allows an overall transmission factor to be determined for each individual path which leads to an antenna element, which allows a weighting process to be carried out and, in the end, allows phase variation.
The overall complexity in this case is also considerable, since each antenna column must have an associated directional coupling device. A coupling device is required in this case since, as mentioned, on the one hand one signal element is masked out via this in each individual signal path and, on the other hand, a signal element which arrives via the reflection device and the power splitter must be injected once again in each individual path via the directional couplers that are provided, in order to carry out the appropriate evaluation.
The present exemplary illustrative non-limiting implementation provides a calibration device for an antenna array, as well as an associated antenna array, which is of simple construction and at the same time has advantages over the prior art. The antenna array according to the exemplary illustrative non-limiting implementation is in this case preferably intended to be a dual-polarized antenna array. The associated calibration device should therefore preferably be suitable for a dual-polarized antenna array of this type.
The calibration device and antenna array according to the exemplary illustrative non-limiting implementation are distinguished by numerous simplifications.
An exemplary illustrative non-limiting implementation now makes it possible to provide fewer probes or coupling devices for each column of an antenna array having two or more antenna elements arranged one above the other, that are provided in the relevant column of the antenna array with antenna elements which are arranged one above the other. When N antenna elements or coupling devices are arranged one above the other in each case, the exemplary illustrative non-limiting implementation makes it possible without any problems to provide, for example, only N/2 fixed probes per column.
In an exemplary illustrative non-limiting implementation, once again with N antenna elements arranged one above the other, only a single fixed probe is required per column, via which both polarizations can be measured. If, for example, a coupling device in the form of a directional coupler is used, two coupling devices are preferably used for a dual-polarized antenna element (i.e., one coupling device for each polarization).
According to the exemplary illustrative non-limiting implementation, it is possible to provide only two fixed probes (or two fixed coupling devices for a single-polarized antenna array or, for example, two pairs of fixed coupling devices for a dual-polarized antenna array) for an antenna array having, by way of example, four columns, with these probes preferably being arranged symmetrically with respect to the vertical central plane of symmetry. It is thus possible, for example, to provide in each case one probe (or in each case one coupling device in the case of a single-polarized antenna array or in each case one pair of coupling devices for a dual-polarized antenna array) for the two outermost columns or, for example, to provide in each case one probe (or, once again, the coupling device in a corresponding manner) for the two central columns.
In the case of an exemplary illustrative non-limiting beam forming network which is preferably in the form of a Butler matrix, it is possible to use only one, but preferably at least two, fixed probes, which are each associated with one antenna element in a different column of the antenna array. The measurement results obtained in this way make it possible to determine a phase relationship between all the antenna elements. In the end, this is possible because the individual antenna elements (their arrangement as well as the length of the feed cables for a connection point on the input side) can be measured and matched as far as the antenna elements such that all the antenna elements also emit with a fixed predetermined phase relationship with respect to one another when using a beam forming network, for example, in the form of a Butler matrix. If any phase shifts occur as a result of upstream beam forming networks or as a result of different upstream cable lengths, then the phase shifts caused by them act on all the antenna elements so that, in the end, a shift in the phase angle can be detected via only a single fixed probe or, possibly, only by a single coupling device that is associated with an antenna element. This is true even when a down tilt angle is preset or provided for the large number of antenna elements in the antenna array.
The test signals for the calibration process are, in one exemplary illustrative non-limiting implementation, not tapped off via coupling devices such as directional couplers, but rather via probes which may be provided in the near field. In this case, it has been found to be particularly advantageous that only a single probe is required for both polarizations, even for dual-polarized antenna elements. The probes may be arranged such that they are positioned directly on the reflector plate of an antenna array, such that the vertical height extent measured from the plane of the reflector plate is less than the position and arrangement of the antenna elements, for example of the dipole structures for the antenna elements. The calibration device and antenna array according to the exemplary illustrative non-limiting implementation, may also be formed from patch antenna elements or from combinations of patch antenna elements with dipole structures.
For example, in one exemplary illustrative non-limiting implementation, the small number of probes which are provided for each antenna array column or, for example, a single probe that is provided for a number of columns, is or are preferably arranged on the uppermost or lowermost antenna element, or on the uppermost or lowermost dipole antenna element structure. A corresponding situation occurs when coupling devices are used instead of the probes. The probes are preferably arranged in a vertical plane at right angles to the deflector plane and running symmetrically through the dual-polarized antenna element structure. However, in principle, a lateral offset is also possible.
The preferably at least two capacitive or inductive probes or the coupling devices which may be used are permanently connected to one another by means of a combination network. This combination network is preferably designed such that the group delay time from the input of the respective column to the output of the combination network has an approximately equal magnitude for all the antenna inputs (at least with respect to one polarization for dual-polarized antennas), and over the entire operating frequency band.
A further improvement can also be achieved by the combination network containing lossy components. This is because these components contribute to reducing resonances.
The antenna array according to the exemplary illustrative non-limiting implementation and/or the calibration device according to the exemplary illustrative non-limiting implementation are/is suitable for calibration of an antenna array in which the antenna elements and groups of antenna elements which are arranged in the individual columns are normally each driven via their own input. An appropriate phase calibration can thus be carried out by means of the calibration device according to the exemplary illustrative non-limiting implementation, in order to obtain a desired beam shape. In this case, it is likewise possible to provide for the main beam direction to be swiveled, particularly in the azimuth direction (or else, of course, in the elevation direction). The antenna array according to the exemplary illustrative non-limiting implementation and the calibration device according to the exemplary illustrative non-limiting implementation may, however, also be used just as well if the antenna array is preceded by a beam forming network, for example in the form of a Butler matrix.
The phase angle of the transmission from the input of the individual columns or of the antenna inputs is admittedly preferably of the same magnitude but, in practice, the phase angle (or the group delay time) is subject to discrepancies from the ideal phase angle to a greater or lesser extent, due to tolerances. The ideal phase angle is that for which the phase is identical for all the paths, to be precise even with respect to the beam forming. The discrepancies to a greater or lesser extent which are due to tolerances occur additively as an offset or else as a function of frequency, due to the different frequency responses. According to the exemplary illustrative non-limiting implementation, provision is in this case made for the discrepancies across all the transmission paths to be measured, preferably on the path from the antenna array or beam forming network input to the probe output, or from the input to the probe outputs, and preferably over the entire operating frequency band (for example during the production of the antenna). When using coupling devices, the transmission paths are preferably measured on the path between the antenna array or beam forming network input and the coupling output or coupling outputs. This data determined in this way is then stored in a data record. This data, which is stored in suitable form, likewise for example in a data record, can then be provided to a transmission device or to the base station in order then to be taken into account for producing the phase angle of the individual signals electronically. It has been found to be particularly advantageous, for example, to associate this data, or the data record that has been mentioned, with the corresponding data for a serial number of the antenna.